1. Field of the Invention
The present invention generally relates to stages and, more particularly, to the positioning of a counter mass to dampen vibrations caused when the stage is driven.
2. Description of the Prior Art
When a semiconductor element is manufactured, a projection exposure apparatus is used to transfer an image of a pattern (reticle), used as a mask, onto areas of a resist coated wafer. Two common types of projection systems include batch exposure type step-and-repeat optical projection apparatuses (stepper), and scanning exposure type projection exposure apparatuses (step-and-scan type), which perform an exposure as the reticle and the wafer are synchronously scanned with respect to an optical projection system.
In conventional systems, such as steppers and step and scan systems, for example, the reticle stage which supports and carries the reticle having the original pattern, the wafer to which the pattern is to be transferred, and the driving part of the wafer stage, are fixed to a structural body that supports the optical projection system. In order to position a wafer stage with high accuracy, the position of the wafer stage is measured by a laser interferometer, and a moving mirror for the laser interferometer is fixed to the wafer stage.
As described above, in the conventional systems, the driving part of the wafer stage or the like and the projection optical system are fixed to the same structural body. The vibration generated by the driving reaction of the stage is transmitted to the structural body, and the vibration is also transmitted to the optical projection system which can cause position shifting of the transfer pattern image and deterioration of contrast.
To minimize this vibration problem, many lithography machines have a moving counter-weight (or counter mass) to preserve the location of the center of gravity of the stage system during any stage motion by using the conservation of momentum principle. As disclosed, a drive assembly includes a drive frame carrying linear motors suspended above a base structure. When the drive assembly applies an action force to the stage to move the stage in one direction over the base structure, the driving frame, acting as the counter weight, moves in the opposite direction in response to the reaction force to substantially maintain the center of gravity of the whole stage apparatus. This apparatus essentially eliminates any reaction forces between the stage system and the base structure on which the stage system is mounted, thereby facilitating high acceleration while minimizing vibrational effects on the system.
Recently, modem wafer stages require larger driving forces. For example, if the stage weighs 180 kg, the driving force required to move this stage at the required acceleration (2 g's) and speed is 2*9.8 m/s2*180 kg=3528 N. For such a large force, the reaction force shakes the system, and degrades the system accuracy. To avoid this phenomenon, the wafer exposure system may use a counter mass as noted above.
It is thus realized that modem lithographic machines of suffer from many problems. These problems range from stability requirements and more particularly vibrations and shaking of the wafer stage with use of large driving forces. The shaking and vibrations lead to system degradation including poor lithographic processing of wafers. The present state of the art, however, is not designed to compensate for these inefficiencies.